Push-to-Talk (PTT) is a voice communication service that allows a group to be interconnected using a “walkie-talkie” style of voice communications. Police, fire fighters, and emergency workers have communicated among themselves using PTT technology in public safety wireless networks for many years. Also, PTT is being used in commercial wireless networks, e.g., wireless networks operated by carriers such as AT&T, Verizon Wireless, Sprint Communications, Vodafone Group, etc., with diverse applications ranging from a small group of friends using a PTT conference to determine where to meet to a business supporting a fleet of cab drivers in a metropolitan area.
With push-to-talk, a member of a group speaks to all other group members simultaneously. Illustratively, when the member of the group wants to speak to the other group members, the member “pushes a button” on a handset, speaks, and releases the button. A Request message requesting permission to speak may then be sent from the handset to a PTT server. A floor control mechanism in the PTT server arbitrates all requests to speak in the event that two or more group members attempt to speak at the same time. The floor control mechanism either grants or denies the request by sending a Grant message or a Deny message as a response to the member. Upon granting the request, the floor control mechanism sends a floor Taken message to other members of the talk group to indicate that the floor has been granted to the member. When the member has been granted the floor, i.e., receives permission to speak, the audio signal of the member is transmitted to the PTT server, which replicates and distributes the packets to all other talk group members. This is known as a talk burst. Each PTT call consists of one or more talk bursts. When the member has finished speaking, the member releases the handset button. A floor release message, i.e., End message, is generated to notify the PTT server that the speaker has finished speaking, and has released the floor. The PTT server sends an idle message to all other talk group members to notify them that the floor is available. In some instances, the PTT server may send a Revoke message to a current floor owner to indicate that floor ownership is revoked.
The PTT server is responsible for ensuring that only one member speaks at a time and that the speech is distributed to all other group members who are authorized to listen. A PTT server may handle the communication needs of multiple talk groups simultaneously. The PTT server allocates resources for calls and de-allocates resources when they are no longer needed.
Commercial wireless networks do not support simultaneous voice transmissions within a PTT talk group, however, support of simultaneous audio streams in commercial systems being used in law enforcement is necessary. Public safety wireless networks are capable of providing limited support for simultaneous voice transmissions within a PTT talk group. The PTT server may designate one stream of audio from the client that has been granted the floor as “preferred”, while other audio streams may be designated as “losing audio”. Typically, preferred audio is broadcasted over the air to all of the recipients in a talk group, while losing audio is preconfigured to be sent only to pre-selected terminals, such as dispatcher consoles and logging devices. Illustratively, a call may be preempted by a higher priority call. However, the preempted call may still be important. The PTT server may allow the preempted member to continue speaking, but may send the audio to dispatchers and logging devices only. Also illustratively, an emergency call may lose in floor arbitration because another emergency call has the floor. The PTT server may direct the losing emergency call to the dispatchers and logging devices. Further illustratively, PTT control messages may become lost in the network. A handset could request the floor, but may not receive a response from the PTT server. As message delay is a major concern for public safety, many systems may wait for a short time and then allow the user to speak even without a Grant message. This is commonly referred to as “early audio”. The PTT server may treat this early audio as losing audio if some other member has been granted the floor.
Disadvantageously, PTT control is specified for voice services only for simultaneous transmissions within a talk group in public safety networks. Also disadvantageously, the originator of the media stream, i.e., a handset, determines when to send simultaneous communications, which results in the PTT server reacting to the decision of the handsets in public safety networks. Further disadvantageously, simultaneous transmissions within a talk group in public safety networks are specified for specific narrow band technologies, i.e., Association of Public Safety Communications Officials (APCO) Project 25 networks (P25 networks). Thus, the PTT control messages are completely specified.